The essence of coaching is teaching. Mel Tucker has built a 17-year career on that core belief. It guides him during prosperity and through adversity. And for the Bears' defensive coordinator, these are trying times.

His injury-riddled defense surrendered 261 total rushing yards to a trio of Rams rookies last Sunday in a performance that stands as the latest candidate for low point of the season.

In reviewing the carnage, one element in particular perplexed Tucker and his fellow coaches.

Runs they defended properly in practices preceding the game gashed the Bears. Somewhere between the secluded Walter Payton Center in Lake Forest and the raucous Edwards Jones Dome in St. Louis, there was a disconnect.

Tucker's students studied well, but they failed the test. His lessons ultimately did not get through.

"As a coach, it's heartbreaking when they don't succeed," he said. "The first question you ask yourself is, 'What can I do to help them be better?' And the players are doing the same thing. That's where we meet. You come on the practice field, and they're determined to get better and we're determined to help them."

So there was Tucker on Wednesday, standing at the 30-yard line inside the Payton Center with a rolled-up practice script in his left hand.

He watched his defensive linemen practice exploding low out of their stances. In front of him, a team videographer recorded the drill for coaches' review. Between defensive line snaps, he turned to the adjacent section of the field and watched his linebackers practice pass reads.

Tucker fully grasps the magnitude and urgency of the challenge he faces as the Bears push for a postseason berth, beginning with Sunday's apparent mismatch against Vikings running back Adrian Peterson, the NFL's reigning MVP.

Injuries have decimated the defense. Only two starters in the front seven filled the same positions against the Rams as they did on opening day. Inexperienced players are filling key roles, as a result, and underachievement is a hindrance in other areas.

Tucker is relying on his mantra as he forges ahead. Teaching — relentless teaching, to be exact — is the only path to a turnaround.

"There's no magic," Tucker said. "You can't sprinkle any dust on anybody. It's just a day-to-day deal. And when it does come together in a game, it looks good. When it doesn't come together, it looks bad. There's no in between."

Defense in transition

Tradition and pride cloak the Bears' defense, which make this season's sorry statistics awkward and unsettling. And now that the Bears have the potent offense for which they have pined for years, the defensive results are excruciating proportionately.

They rank 30th in the NFL in yards allowed per run (4.9). They're 28th in yards per play (6.0) and 27th in sacks per pass attempt (5.6 percent). They're on pace to give up more than 6,000 yards for the first time in franchise history.

Tucker's role in this season's disappointment is debatable, mostly because of injuries to Pro Bowlers Lance Briggs, Henry Melton and Charles Tillman and starters D.J. Williams and Stephen Paea.

He's the first to say he hasn't done well enough.

"Nobody cares about (injuries) because the bottom line is: Did we stop them, and did we win?" he said. "That's the only thing that matters."

Bears coach Marc Trestman, however, looked past the numbers.

"Mel has done an amazing job, in my opinion," Trestman said last month. "He has been consistent with the guys in terms of his passion, the way he's teaching.

"Are we where we want to be? No. We're always trying to get better. But he has done an outstanding job in the classroom. On the field, he's excellent."

It's clear Trestman sees a picture broader than just this season. Considering six of the 11 defenders projected to start Sunday are not under contract for 2014, the Bears' defense is in transition.

Under second-year general manager Phil Emery, there's a heavy organizational emphasis on player development. That's evident in how many reserves and inexperienced players, such as second-round rookie middle linebacker Jon Bostic, stay after practice to work on reads, alignments and techniques.

"When I do my (weekly media session) Wednesday, win or lose, I'm usually in a good mood," Tucker said. "The reason is not just because I'm a happy-go-lucky guy. That's not me. It's because we've come off a practice, and I've seen guys since they got here at 6:30 in the morning, and sometimes guys are still out there working. I'm saying, 'You know what? We're making some progress here.' "

That sense is challenged, though, by games such as Sunday's. Tucker reviewed the video and saw players failing to apply lessons they previously had discussed.

On one play, a defender crept too close to the line of scrimmage before the snap and was in too tight to play the ball carrier laterally. On a touchdown, someone tried to go underneath a block instead of over it and was out of position.